Sludge treatment processes using enzymes have been known for a number of years to permit a significant shortening of stabilization periods as compared to conventional processes. The patent literature describes various processes relating the degradation of organic matter by the use of a quantity of enzymes depending on the mass of dry matter in the sludge. The known enzymatic processes aim at an as rapid as possible degradation of the activated sludge into a stabilized form. Herein, small quantities of added enzymes are intended to bring about high degradation rates and also a favorable impact on the dewatering characteristics of enzymatically treated sludge.
In a process according to British Pat. No. 1,563,335, 0.001 to 15% of finely comminuted dry matter, dry enzymes in a quantity of up to 5% based on dry matter, and thermophilic microorganisms are fed into a reaction vessel and are aerated, while the sludge is continuously maintained at a temperature of from 50.degree. to 70.degree. C.
German Federal Republic Pat. No. 2,633,514 relates to a process for increasing the filterability and the capacity of sedimentation of suspensions, wherein an enzyme or a group of enzymes is added to the suspension under agitation.
U.S. Pat. No. 4,267,049 describes the transformation of treated sludge into reusable substances by hydrolysis of the organic matter in the sludge with the addition to the sludge of at least one hydrolytic enzyme, and microorganisms producing hydrolytic enzymes, by leaving at least one sludge to itself with enzymes or microorganisms, and by subsequent separation of the suspension into an effluent and a moist product.
In German Democratic Republic Pat. No. 234,571 there is disclosed a process wherein the organic matter to be enzymatically treated and the biomass are fed into a reaction medium containing, depending on the kind and composition of the organic matter or the kind of biomass to be treated, 0.01 to 1% wt., based on the mass of organic dry matter, of added hydrolytic enzymes, and 0.005 to 0.5% wt. of a chelating agent, at a temperature of 30.degree. to 60.degree. C.
The reaction medium is a watery solution of already enzymatically treated organic substances, such as sewage sludge or liquid manure, or activated sludge biomass, or microorganism cultures.
Upon introduction of the organic substances, including biomass into this reaction medium, the hydrolytic enzymes present start a quick hydrolytic degradation of organic macromolecules such as carbohydrate, proteins, fats contained in the organic substance including biomass, into microbially easily degradable low-molecular weight compounds, such as sugars, and organic acids. The latter compounds are immediately available for disposal by the microorganisms added as biomass or in other organic substances added to the reaction medium, thus stimulating the biochemical activity of the microorganisms. The chelating agent contained in the reaction medium has an impact on the metabolism of the microorganisms insofaras it triggers by its permeability increasing effect, a strong one-sided promotion of their respiration, and accelerates the exchange of substances through their cell walls.
The temperature shift is caused by the sudden supply of the organic substances including biomass having at least 10.degree. C. lower temperature into the reaction medium that is at 30.degree. to 60.degree. C. This triggers an intensified release of exoenzymes by microorganisms into the reaction medium as well as processes of autolysis and sporulation of microorganisms. The joint effect of hydrolytic enzymes, the chelating agent and the temperature shift leads to a quick degradation of biodegradable substances and biomass and to an intensified exchange of substances across the cell walls of the microorganism as well as to a higher production of metabolites such as enzymes, vitamins, organic acids, antibiotics, etc.
Apart from natural dewatering, centrifugal dehydration as one method of mechanical dehydration is known to be applied to sewage sludge. Considerable quantities of polymeric flocculants are required to obtain a relatively high solids concentration in the output of centrifuges and a minimal concentrate concentration.
Up to now, evidence on the dewatering behavior could be only obtained by expensive large-scale dehydration tests. The so far stated laboratory parameters, such as head loss, CST have only permitted limited assessment of the dewatering behavior. Granulometric measurements and recording of the surface potential, however, have led to conditions which permit assessment of the result of dewatering.
The use of expensive flocculants frequently does not lead to a dry matter content of the output that would permit subsequent incineration without added fuel.
Incineration reduces the sludge volume to a minimum. The remaining ashes are sterile and can simply be discarded. State of the art incineration processes use both fresh activated sludge mostly requiring significant amounts of added fuel and also digested sludge.
The extra energy of the fuel additive is needed to incinerate digested sludge when the water content of the sludge cake is too high. Initial operation will, in any case, require added fuel. The considerable cost of the added fuel and energy make state of the art technology of incineration a rather costly process which can, however, not be done without in view of the present and future lack of landfill space.
According to German Federal Republic Pat. No. 3,429,055 a process for the disposal of sewage sludge is provided wherein a portion of the thick sludge is predried to a predetermined dry matter content, and the remaining portion of thick sludge is incinerated as the dry product is continuously added.
German Federal Republic Pat. No. 2,916,216 relates a process for the reuse of sludge wherein the sludge is thickened, conditioned by addition of fine coal, fed to a filter press, and then incinerated.
The disadvantage inherent in the treatment of sludge in its preparation for incineration are only partly alleviated by the last mentioned processes.